In the art of container closures, it is well known to provide product packaging with tamper-evident closure systems which are intended to reveal, upon visual inspection, whether the package has been previously opened. Such tamper-evident packaging considerations have recently assumed increased importance and have become the subject of considerable attention owing to various instances of intentional tampering with consumer products such as pain relievers and foodstuffs.
Tamper-evident closures, for bottles and jars especially, have most commonly been provided as formed aluminum or molded plastic screw caps. In general, all such closures engage a bottle or other container with a locking engagement sufficient to prevent disengagement of the closure without destruction of at least part of the lock portion of the closure. One example of a plastic tamper-evident closure system is a screw cap closure which closes the mouth of a container, such as a bottle or jar, and which includes an annular tear-away band attached to the lower extremity of the screw cap skirt by frangible bridges. The band typically is formed to engage a flange portion of the jar or bottle during first opening thereof to break the frangible bridges, thus separating the band from the cap skirt. Upon subsequent closing of the container, the fractured bridges provide visual indication that the container has been previously opened.
Plastic tamper-evident closures comprise two general types, a shrink-fitted type having a band which is shrunk around the container after the closure has been applied (see U.S. Pat. No. 4,349,399, for example) and a mechanical lock type having a band which provides for locking engagement with the container as the closure is applied thereto. Shrink-fitted closures typically have been formed of thermally-sensitive plastic material which, when subjected to heating, shrinks into tight-fitting engagement with the container to which it has been applied. Without such heating, the shrink-fit closure will not form the requisite tight-fitting engagement for tamper indication. Also, the shrink-fit cap locking structure typically is not subjected to any significant degree of mechanical loading during initial cap application.
The mechanical lock type of tamper-evident closure has been perceived as an improvement over heat-activated, shrink-fit closure systems because it requires no heat input and is, therefore, more economical to install; however, mechanical lock closures have been subject to other diseconomies and structural limitations which limit the economic advantage thereof over alternative tamper-evident closure systems. For example, during application of a mechanical lock closure to a container, the frangible portion thereof must deform, without self-destructing, to override the corresponding lock portion of the container. Any destruction of the closure locking elements (e.g., vertically extending bridges or vertical score lines) during closure application will render the intended tamper-evidencing capability thereof essentially useless. This results in significant quantities of defectively capped packages and many wasteful recapping operations to correct the defects.
Designing the frangible bridges or other such elements to withstand greater deformation upon closure installation introduces undesirable diseconomy of material use, limits the available choices for closure material and design specifications, and may result in a closure which is difficult to remove from its container, or whose tamper-evidencing structures will not readily fail on closure removal. For example, polypropylene, which is one commonly used material for mechanical lock type caps, is quite sensitive to high force loading. In general, it will absorb a magnitude of loading if the load is applied slowly, but will shatter if the same load is applied quickly. Thus, the design parameters of a polypropylene cap will have a direct impact upon the permissible rate of cap application for any given capping operation.